1. Field of the Invention
The present invention relates to a magnetic recording medium which is employed in, for example, hard disk devices, to a process for producing the magnetic recording medium, and to a magnetic recording and reproducing apparatus.
2. Background Art
The recording density of a hard disk device (HDD), which is a magnetic recording and reproducing apparatus, has increased at a rate of 60% per year, and this tendency is expected to continue. Therefore, magnetic recording heads and magnetic recording, media which are suitable for attaining high recording density are now under development.
Magnetic recording media employed in hard disk devices are required to have high recording density, and therefore demand has arisen for improvement of coercive force and reduction of medium noise.
Most magnetic recording media employed in hard disk devices have a structure including a magnetic recording medium substrate on which a metallic film is laminated through sputtering. Aluminum substrates and glass substrates are widely employed for producing magnetic recording media. An aluminum substrate is produced through the following process: an Ni—P-based alloy film (thickness: about 10 μn) is formed through electroless plating on an Al—Mg alloy substrate which has undergone mirror polishing, and the surface of the Ni—P-based alloy film is subjected to mirror polishing. Glass substrates are classified into two types; i.e., amorphous glass substrates and glass ceramic substrates. When either of these two types of glass substrate is employed to produce a magnetic recording medium, the substrate is subjected to mirror polishing.
In general, a magnetic recording medium employed to produce a hard disk device includes a non-magnetic substrate; a non-magnetic undercoat layer (formed of, for example, an Ni—Al-based alloy, Cr, or a Cr-based alloy); a non-magnetic intermediate layer (formed of, for example, a Co—Cr-based alloy or a Co—Cr—Ta-based alloy); a magnetic layer (formed of, for example, a Co—Cr—Pt—Ta-based alloy or a Co—Cr—Pt—B-based alloy); a protective film (formed of, for example, carbon), the layers and film being successively formed on the substrate; and a lubrication film containing a liquid lubricant formed on the protective film.
The Co—Cr—Pt—Ta-based alloy or the Co—Cr—Pt—B-based alloy employed in the magnetic layer contains Co as a primary component. Such a Co alloy has a hexagonal closest-packed (hcp) structure in which the C-axis is an easy-magnetization axis. Magnetic recording media are classified into a longitudinal recording type and a perpendicular recording type, in which the magnetic layer is generally formed of a Co alloy. In a longitudinal recording medium, the C-axis of a Co alloy is oriented horizontally with respect to the non-magnetic substrate, and in a perpendicular recording medium, the C-axis of a Co alloy is oriented perpendicular to the non-magnetic substrate. Therefore, in a longitudinal recording medium, desirably, crystals of a Co alloy are oriented along a (10•0) plane or a (11•0) plane.
As used herein, the symbol “•” of the crystal plane notation refers to the abbreviation of a Miller-Bravais index. In general, crystal planes of Co (hexagonal system) are represented by four indices (hkil). Of these indices, the index “i” is defined as follows: i=−(h+k). Therefore, the indices (hkil) can be abbreviated as (hk•l).
In a perpendicular recording medium, desirably, crystals of a Co alloy are oriented along a (00•1) plane. However, in a longitudinal recording medium, when crystals of a Co alloy are oriented along a (10•1) plane or a (00•1) plane; i e., a crystal plane perpendicular to the substrate, reduction of magnetization in a longitudinal direction is induced, which is not preferable.
Since difficulty is encountered in directly orienting crystals of a Co alloy along a (10•0) plane or a (11•0) plane, in general, a Cr alloy having a cubic body-centered (bcc) structure is employed in an undercoat layer. The (11•0) plane of a Co alloy tends to be oriented along the (100) plane of a Cr alloy, and the (10•0) plane of a Co alloy tends to be oriented along the (112) plane of a Cr alloy.
When an aluminum substrate on which an Ni—P-based alloy film has been formed through electroless plating is heated, and then a Cr alloy film is formed on the substrate, crystals of the Cr alloy tend to be oriented along a (100) plane. When a Co alloy film is epitaxially grown on the Cr alloy film, crystals of the Co alloy are oriented along a (11•0) plane, and the resultant magnetic recording medium exhibits excellent properties.
Meanwhile, when a glass substrate is heated, and then a Cr alloy film is formed directly on the substrate, crystals of the Cr alloy tend to be oriented along a (110) plane. Therefore, when a Co alloy film is grown on the Cr alloy film, crystals of the Co alloy are oriented along a (10•1) plane, which is not preferable. When crystals of the Co alloy are oriented along a (10•1) plane, the C-axis (i.e., easy-magnetization axis) of the Co alloy has vector components in both the longitudinal and perpendicular directions, and thus the resultant magnetic recording medium is suitable as neither a longitudinal recording medium nor a perpendicular recording medium.
There have been proposed various techniques for orienting crystals of a Cr alloy film formed on a glass substrate along a (100) plane or a (112) plane.
In a technique proposed in European Patent Application EP 0704839 A1 employing an Al alloy having a B2 structure (e.g., an Al—Ni, Al—Co, or Al—Fe-based alloy) in an undercoat layer, crystal grains of a magnetic layer are confirmed to become small by the effect of an Al alloy such as an Al—Ni-based alloy or an Al—Co-based alloy, whereby noise is reduced. Among Al alloys having a B2 structure, an Al—Ni-based alloy is widely employed as a material for a non-magnetic undercoat layer. This is because the (112) plane of an Al—Ni-based alloy establishes excellent lattice matching with the (10•0) plane of a Co alloy in a magnetic layer, and therefore the (10•0) plane of the Co alloy is epitaxially grown on the (112) plane of the Al—Ni-based alloy. As a result, crystals of the magnetic layer formed of the Co alloy are oriented along the (10•0) plane, and thus high coercive force is obtained.
Japanese Patent No. 3217012 discloses a technique in which an undercoat layer predominantly containing Co is formed beneath a Cr alloy film, to thereby grow a Cr (100) plane and promote epitaxial growth of the (11•0) plane of a Co alloy.
In the technique proposed in European Patent Application EP 0704839 A1 employing an Al alloy having a B2 structure (e.g., an Al—Ni, Al—Co, or Al—Fe-based alloy) in an undercoat layer, crystals of an Al—Ni-based alloy may be insufficiently oriented along the (112) plane of the alloy, since the peak of the (112) plane is small. Therefore, as described in this publication, in order to orient crystals of the Al—Ni-based alloy along the (112) plane, the thickness of the Al—Ni-based alloy layer must be increased. However, when the thickness of the layer is increased, crystal grains of the Al—Ni-based alloy become large. When an Al—Ni-based alloy is employed in an undercoat layer, in order to attain high coercive force, the thickness of the layer must be increased. Meanwhile, in order to reduce the size of crystal grains so as to lower medium noise, the thickness of the layer must be decreased. Therefore, the aforementioned technique encounters difficulty in designing an optimal layer structure for a magnetic recording medium.
As described in Examples of Japanese Patent No. 3217012, when a Co alloy (e.g., Co-30at % Cr-10at % Zr, Co-36at % /cMn-10at % Ta, Co-30at % Cr-10at % SiO2, or Co-25at % Cr-12at % W) is employed in an undercoat layer, crystals of a Cr alloy are oriented along a (100) plane, and epitaxial growth of the (11•0) plane of the Co alloy is observed. However, the Cr alloy crystals arc insufficiently micronized, and a limitation is imposed on reduction of medium noise.
In association with an increase in recording density of magnetic recording and reproducing apparatuses, demand has arisen for reduction in the flying height of a magnetic head. In order to reduce the flying height of a magnetic head, the following two methods (the CSS method and the ramp load method) are generally employed.
In the CSS (contact start and stop) method, when a disk is stopped, the disk is in contact with a magnetic head, and when the disk is rotated, the head flies over the disk by means of pressure generated by airflow above the disk. In the CSS method, when appropriate irregularities (protrusions and depressions) are not present on the surface of the disk, adhesion between the head and the disk occurs. However, when the irregularities are formed on the entire surface of the disk, the flying head may hit the protrusions, and thus the flying height of the head may fail to be reduced. Therefore, in general, such irregularities are formed on a portion of the disk surface. The portion is called a “CSS zone,” at which the head and the disk come in contact wish each other when the disk is stopped. Data are recorded in the remaining portion of the disk. In order to form irregularities merely on the CSS zone, typically, laser machining is employed. In the CSS method, generally, an aluminum substrate is employed, since such a substrate is readily subjected to laser machining.
In the ramp load method, when a disk is stopped, a magnetic head is moved to a ramp provided at the periphery of the disk. In the method, formation of irregularities on the surface of the disk is not required, and thus the flying height of the head can be reduced. In the ramp load method, generally, a glass substrate, on which irregularities are difficult to form through laser machining, is employed. The ramp load method is ideal in that formation of irregularities on the surface of the disk is not required, but the method involves a problem in that the head hits the disk when the head moves from the ramp to the surface of the disk. Therefore, damage (e.g., film exfoliation) is likely to arise at the boundary between the ramp and the disk. In order to avoid such a problem, the ramp or the head must be designed appropriately, and the disk is required to employ a film exhibiting good adhesion to the glass substrate.